Conventionally, MRI has been used to produce images by exciting the nuclei of hydrogen molecules (present in water protons) in the human body. However, it has recently been discovered that polarized noble gases can produce improved images of certain areas and regions of the body that have heretofore produced less than satisfactory images in this modality. Polarized Helium 3 (“3He”) and Xenon-129 (“129Xe”) have been found to be particularly suited for this purpose. See U.S. Pat. No. 5,545,396 to Albert et al., entitled “Magnetic Resonance Imaging Using Hyperpolarized Noble Gases”, the disclosure of which is hereby incorporated by reference herein as if recited in full herein.
In order to obtain sufficient quantities of the polarized gases necessary for imaging, hyperpolarizers are used to produce and accumulate polarized noble gases. Hyperpolarizers artificially enhance the polarization of certain noble gas nuclei (such as 129Xe or 3He) over the natural or equilibrium levels, i.e., the Boltzmann polarization. Such an increase is desirable because it enhances and increases the Magnetic Resonance Imaging (“MRI”) signal intensity, thereby potentially allowing physicians to obtain better images of many tissues and organs in the body.
Generally stated, in order to produce the hyperpolarized gas, the hyperpolarizer is configured such that the noble gas is blended with optically pumped alkali metal vapors such as rubidium (“Rb”). These optically pumped metal vapors collide with the nuclei of the noble gas and hyperpolarize the noble gas through a phenomenon known as “spin-exchange”. The “optical pumping” of the alkali metal vapor is produced by irradiating the alkali-metal vapor with circularly polarized light (typically provided by lasers) at the wavelength of the first principal resonance for the alkali metal (e.g., 795 nm for Rb). Generally described, the ground state atoms become excited, then subsequently decay back to the ground state. In the presence of non-zero nuclear spin noble gases, the alkali-metal vapor atoms can collide with the noble gas atoms in a manner in which the polarization of the valence electrons is transferred to the noble-gas nuclei through a mutual spin flip “spin-exchange”.
After the spin-exchange has been completed, the hyperpolarized gas is separated from the alkali metal prior to introduction into a patient.
Conventionally, gas-phase imaging has been possible using both 3He and 129Xe, and has been particularly useful in producing ventilation-driven images of the lungs, a region where proton images have produced signal voids. However, in contrast to gas phase imaging, dissolved phase imaging has proven to be problematic. Dissolved phase imaging uses the solubility characteristic of 129Xe in blood and lipid rich tissue. The gas phase is thus absorbed or “dissolved” into surrounding tissue or blood vessels and may allow perfusion imaging of the brain, lung, or other regions. Such images can potentially allow for the performance of non-invasive studies of the pulmonary vasculature to detect emboli and other circulatory system problems. Unfortunately, once the polarized gas has been dissolved (such as into the blood vessels), it has proven difficult to generate clinically useful images using the dissolved phase gas.
For example, MRI images using gas-space-imaging techniques have been generated using hyperpolarized 129Xe gas. See Mugler III et al., MR Imaging and Spectroscopy Using Hyperpolarized 129Xe gas: Preliminary Human Results, 37 Magnetic Resonance in Medicine, pp. 809–815 (1997). While good correlation is seen between the gas-space signal in the xenon images and the gas-space signal void in the proton images, the spectra associated with the dissolved phase signal components were significantly lower than the gas-phase signal.
There remains a need to provide clinically useful methods for using polarized gas to perform in vivo evaluations of the body.